Multiple stage analog-to-digital (A/D) converters are a cross between successive approximation converters and flash A/D converters wherein multiple flash A/D converters and one or more digital-to-analog (D/A)/ converters are used to achieve the conversion. For example, two-stage, or subranging, A/D converters use a first stage estimator A/D converter to make a rough estimate of the size of the analog input signal. The digital output of the estimator A/D converter drives an estimator D/A converter. The analog output of the estimator D/A converter is subtracted from the analog input signal to provide a residue voltage having a smaller amplitude. The residue voltage is amplified and applied to a second stage residue A/D converter that provides a digital output representing the analog residue voltage. The digital output of the residue A/D converter is added to the digital output of the estimator A/D converter along with the proper bit shift to account for the gain of the residue voltage amplifier. This two-stage conversion process provides increased resolution.
The accuracy of a subranging A/D converter is limited primarily by the linearity of the estimator D/A converter and the accuracy of the residue amplifier gain. The estimator D/A converter analog output voltage must represent its digital input accurately because it is necessary to know digitally how much analog voltage is being subtracted from the analog input signal. Similarly, the gain of the residue amplifier and residue A/D converter must be known relative to the output of the estimator D/A converter.
Actual performance of A/D converters is degraded from the theoretical ideal by such factors as D/A and A/D converter nonlinearities, residue amplifier gain error, aperture Jitter, timing errors, nonlinear input impedances, amplifier noise, sample-and-hold settling time, digital noise pickup, and code errors due to metastable flip-flops states. All but the first two of these sources of error should be reduced by the smaller geometries of forthcoming semiconductor devices. However, there is a current need for correction of converter nonlinearities and amplifier gain errors in high-speed, wide dynamic range A/D converters used in wideband digital processing of HF, VHF, and UHF radio signals. The desired A/D converters should have discrete distortion products which are 80 to 100 dB below full scale, noise densities that are 140 to 170 dB/Hz below full scale, and sampling rates of from 10 to 100 Ms/s. Because currently available A/D converters cannot meet these specifications, an improvement in the performance of subranging A/D converters has been sought through self-calibration for linearity correction.